Induction furnace coil



July 24, 1962 H. GASSEN INDUCTION FURNACE COIL 3 Sheets-Sheet 1 Filed July 10, 1959 a .Hi

July 24, 1962 H. GASSEN INDUCTION FURNACE COIL 3 Sheets-Sheet 3 Filed July 10, 1959 Jn venfor HORST 6195554 QVJ (9W f) 77' ail 91;

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In furnaces conventionally used for inductive heating under decreased pressure, where an electric coil is arranged in the furnace, electric arcs may be ignited in specific pressure ranges as soon as the coil voltage rises above 130-250 volts, depending upon the particular type of gaseous environment. In this connection, unfortunately, great capacity may only be obtained with safety where high current intensities are employed. Even a comparatively small vacuum type furnace used for melting steel in 25 kilo batches and operating at 250 volts coil voltage, requires about 5,000 amperes, in order to supply 70 kilowatts to the furnace.

To maintain the heat required for a vacuum ladle used for 50 ton steel batches, a furnace capacity of approximately 1,000 kilowatts would be necessary. Thus, if the furnace voltage were limited to 250 volts, then currents of the order of magnitude of 70,000 amperes would be required to attain the necessary Wattage. In practice, however, this can hardly be carried out on an economical and efficient industrial scale.

It is significant with respect to safe operation that electric arcs in vacuum induction furnaces which are produced as a result of higher coil voltages may lead to explosions.

This explosive tendency of electric arcs in vacuum furnaces can be avoided nevertheless by insulation of all current carrying parts against the vacuum. Heretofore, this could be carried out in a manner dependable in operation only with great difficulty. By limitation of the furnace voltage to 130-250 volts, the operation of larger vacuum furnaces, for instance, for the melting of over one ton of steel or for the maintenance in heated condition of more than 10 tons of steel, is rendered extraordinarily difficult.

It is an object of the invention to overcome the foregoing disadvantages and to provide an arrangement of induction coils for vacuum furnaces which makes possible the utilization in the furnace of coil voltages of the order of 1,000 volts and more, heretofore customary only in open furnaces, without the aforementioned electric arc dangers.

Other and further objects will become apparent from a study of the within specification and accompanying drawings, in which:

FIG. 1 is a sectional view, transverse to central axis of an embodiment of an induction furnace in accordance with the invention;

FIG. 2 is a sectional view longitudinally of the central axis of said embodiment;

FIG. 3 is a sectional view similar to that of FIG. 1 of a further embodiment of the invention;

FIG. 4 is a sectional view of a further embodiment of an induction furnace arrangement showing pouring means as well as heat and induction current insulation means;

FIG. 5 is a partial sectional view of a further embodiment of the casing and coil arrangement for an induction furnace illustrating an alternate form; and

FIG. 6 is a partial sectional view similar to that of FIG. 5, wherein the casing is horizontally divided.

It has been found in accordance with the invention that these objects are realized by providing the coil, with which the induction of currents into the metallic material to be treated under vacuum is carried out, in vacuum tight 3,045,320 Patented July 24, 1962 condition on all sides by means of a preferably metallic casing. A transverse slit is defined in each casing wall which nevertheless electrically insulates the voltage and maintains the vacuum tight condition. This slit must be rectangular to the EMP induced in the casing in order to prevent secondary current.

The furnace is generally cylindrical so that the encased coil may be suitably disposed therein. The material to be treated is situated within the core space of the coil. Hence, the slits are preferably approximately defined in the casing Walls parallel to the longitudinal axis of the coil. The sealing of the coil slits may be effected by any suitable means, such as by arranging the corresponding slit edges in flush or overlapping position.

Advantageously, the casing of the coil may be made of a nonmagnetic metal or of an alloy of a nonmagnetic metal or steel.

The power supply lines for the coil are introduced through a conduit extension of the casing passing through the wall of the furnace. This extension is in vacuum tight communication with the main coil casing and is sealed from the furnace proper. Alternatively, the casing may form a part of the power supply line itself, for instance by directly attaching one end of the coil to the casing wall and connecting the casing wall to the current source.

A particular advantage of such arrangements is that the coil may be provided as a simple Wire filament arrangement or litz, i.e., a high frequency conductor made up of a number of strands, each separately insulated and interwoven, and connected together in parallel at the ends, whereby electrical losses may be minimized. This feature is of particular value since the coil and the core space containing the metallic material to be treated, such as molten steel, may possess a diameter which exceeds their height, an arrangement which heretofore was not conveniently possible in conventional induction furnaces. The use of such dimensions, however, is preferred for metallurgical reasons, particularly on account of the better degasification which results during the operation.

FIGS. 1 and 2 are sectional views through the coil of a vacuum furnace in accordance with the invention, transversely and longitudinally to the coil axis respectively. The vacuum chamber 7 of the furnace is cyclindrically arranged and has the same center axis as that of induction coil 1. Coil 1 is completely surrounded by cylindrical casing 2 on all sides and its power supply lines 4 are completely enclosed by the casing extension portion 5. Casing 2 is provided with a slot opening or slit 3, defined in both walls so arranged that no current corresponding to the coil current may flow therethrough. At the slit 3, the two corresponding side wall edges 3d, 3d and 3e, 3e of the casing are connected together with insulated screws 3a in pressing contact with a suitable packing material, for instance, rubber, 3b and 3c, disposed therebetween. Extension portion 5 passes out of the vacuum chamber 7 via opening 6 defined in the outer wall of chamber '7, allowing supply lines 4 to communicate with the current supply source.

By constructing the slit 3 as well as the opening 6 for the extension portion 5 of the casing vacuum tight with respect to the vacuum tank 7, such as by means of rubber or silicon packing material 3b, 30 disposed along the slit 3 in both walls of the casing 2 and rubber or silicon packing material 6n disposed along the opening 6 between extension portion 5 and chamber '7, the space Within the induction furnace may be readily evacuated as desired. As a consequence, the interior of casing 2 containing coil 1 may be kept at any desired pressure, for instance one atmosphere, without affecting the Vac uum conditions in the furnace. The coil voltage may then be maintained just as high as that of conventional 3 open furnaces. Since the electromagnetic field of the coil is not essentially disturbed by the slit covering arrangement, the insert crucible 9 containing the material 8 to be treated may be heated just as easily as if no casing existed.

In accordance with the invention, the support of the crucible is no longer effected by the coil itself, but more advantageously by the casing instead.

The coil itself may be made of any suitable material such as copper pipe and the same may be cooled by circulating water therethrough. The spool conductor or coil may also be made of flexible wire or litz carried within an insulating jacket capable of being cooled with a cooling agent, for instance water, in the same way. The casing, and particularly the slit, may also be conveniently cooled, for instance by circulating water or oil.

The power supply for the coil may be formed as a concentrically arranged cylindrical line. Part of the casing in this case may serve as a suitable outside line. For easier mounting of the coil, a part of the casing, for instance, the top ring-shaped portion 2a may be provided as a removable cover, permitting insertion of the coil thereat. This cover may be suitably sealed against the vacuum furnace by means of a rubber or adhesive sealant. The removable cover of the casing, after the mounting of the coil, may alternatively be Welded or bonded to the remaining portion of the casing. Instead of providing a removable cover, inasmuch as the coil Will not normally be removed after insertion into the casing, the casing may be manufactured with a suitable opening for this purpose, and upon placing the coil within the casing, the same may be sealed in place by welding or soldering.

FIG. 3 shows a preferred form of an induction coil furnace such as that to be used for the melting of 500 kilo batches of steel under vacuum. Coil ll may have, for example, a diameter of 70 cm., a height of 28 cm. and seven coil windings. The coil conductor may be made of copper pipe having a wall thickness of 2 mm. and may be covered with an insulating hose of 1 mm. thickness Coil 1' is situated within the casing 2' having a wall thickness of 3 mm. comprising brass sheeting so that between the coil and the casing there is still suflicient space for cold water to circulate therethrough, especially along the side facing the material to be heated. Casing 2. in this case may have an inside diameter of 68.6 cm., an outside diameter of about 72.4 cm., and a height of 34 cm. At the slit 3 the two corresponding side wall edges of the casing are connected together with insulated screws in pressing contact with a suitable packing material, for instance rubber, disposed therebetween. The upper coil end of the coil conductor is connected via power supply line 4 toward the exterior of the furnace through the tubular appendage 5 of casing 23. The lower coil end is connected for conduction at 21' directly to the casing 2 so that the appendage 5 may act as the second power supply line. Appendage 5' is suitably sealed by packing material 611' at its seating in the opening 6' of the wall of the vacuum furnace 7. On the other side, the projection 9 of the coil is insulatedly seated by packing material 6a in its opening 6 on the furnace wall 7. For support of the crucible 10 which may be made of ceramic material casing 2 carries at its lower end a supporting frame 11. Within crucible 10 lies a block of material to be heated 8, which has a diameter of 56 cm. and a height of 28 cm. If the coil is connected to a 1,000 kilocycle power source with a voltage of 900 volts, the furnace will attain with a current of 6,000 amperes, a capacity of 480 kilowatts.

FIG. 4 is a the longitudinal section through a vacuum furnace which may be used to carry out the degasification of molten metals, such as ton steel batches. By using the induction coil arrangement in accordance with the invention, a decrease in the temperature of the steel during degasification is prevented and the molten material is maintained in agitated condition by the constant attendant bubbling. The coil 1", for instance, may have 38 loops of copper pipe with a cross section of 50 x 20 mm. and the wall thickness of 2 mm. or copper strip with the cross section of 50 X 5 mm. and be adapted to attain, from a power source of 50 cycles/ sec. at a coil voltage of 2,000 volts and a current intensity of 4,600 amperes, a capacity of 1,700 kilowatts. The inside diameter of the coil may be 250 cm., and its height 210 cm. The pipe conductor is suitably separated from the surrounding cooling water provided by a 2 mm. thick insulating layer 12". The casing 2" is made of nonmagnetic steel with a wall thickness of 5 mm. and has an inside diameter of 246 cm., an outside diameter of about 256 cm. and a height of 215 cm. From casing 2 top and bottom pipes 5 conduct the ends 4 of the coil into and out of the furnace. A further pipe 5 provides a conduit for the distribution within the casing space of circulating cooling water which may easily pass through coil and casing and out of the arrangement between top and bottom pipes 5 and ends 4/. Each wall of the casing is provided with a slit opening 3", the edges of the slit being sealed together by suitable packing material and electrically insulated from the furnace. In order to prevent the induction of too high current to the furnace jacket 7, casing 2" and coil 1" are surrounded by a number of L-shaped packets 13', 13" of transformer sheets. The lining of the furnace is rendered fireproof by the provision for magnesite bricks 10" having a wall thickness of, for example, 20 cm. Space 8" is defined within bricks 1.0" which is constructed and arranged to contain the material to be treated, stopper 2% preventing loss through stopper outlet 24 at the bottom of space 8". Below the stopper outlet of the furnace, there is provided a socket 14-" having a lid 15", in order to tightly close off the furnace during the degasification operation. Alternatively, several outlets for the molten material may be provided so that the material may be poured under vacuum into tightly arranged cooling molds 16. Due to the possibility of heating, while pouring, the molten material may be poured very slowly so that very long ingots may be obtained having a small diameter and a structure similar to parts obtained by continuous casting techniques.

In addition, the cooling molds may be suitably water cooled and connected directly to the pouring spouts via sluices.

The furnace may be provided with a lid to which may be advantageously attached pumping nozzles, feed sluices, blowing devices, measuring and observation devices, and the like.

With respect to FIG. 5, a construction is shown in which casing 2 is provided with inlet and outlet means 119" and 20" for a coolant to be circulated within the interior of casing 2". The coil in this embodiment is made up of a litz 17', i.e., a high frequency conductor having a number of strands, in this instance 7 strands, each separately insulated and interwoven, and connected together in parallel at their ends. Because of the particular construction utilizing a litz or litzendraht, the AC. response is less than that of the equivalent cross-section of an analogous solid conductor, owing to the reduction in skin effect. Surrounding the litz coil 17" is an insulation jacket 18" which protects the litz 17" from the circulating coolant which, in this instance, may be water. More importantly, such insulated or insulation jacket 18" serves to insulate electrically the coil 17" from the surrounding wall of casing 2" so that no short circuits Will occur between these elements. It will be seen from this figure that suflicient space is provided between the adjacent turns of coil 17" as well as between said coil and the surrounding casing wall to permit sufficient passage of coolant so as to carry out the desired heat exchange.

Concerning FIG. 6, a similar embodiment is shown to that of FIG. 5, wherein a litz coil 17 enclosed wlthm an insulation jacket 18 is provided within a casing hav- 5 ing cooling inlet and outlet means 19" and 2t)". The casing 2"" in this embodiment, however, is horizontally divided to form an upper portion 2a"" and a lower portion 2b". These portions are connected together by means of strips 23 of sealing or packing material. Of course, in place of strips 23, the connection between the top and bottom portions may be effected by means of soldering or welding together the appropriate edges.

I claim:

1. An induction furnace for heating metallic materials under vacuum conditions by means of induction current which comprises a furnace chamber capable of being maintained under vacuum conditions, an induction coil means disposed Within said chamber, a metallic casing enclosing said coil means in vacuum sealed condition with respect to said chamber and having an open core space axially defined therethrough constructed and arranged to receive therewithin metallic material to be treated, said casing having a transverse slit opening defined therein, said slit opening being in vacuum sealed condition with respect tosaid chamber, and means for supplying induction current to said coil means.

2. An induction furnace according to claim 1 wherein the casing is made of non-magnetic metallic material.

3. An induction furnace according to claim 1 wherein a portion of the casing is provided as one of the means for supplying induction current to said coil means.

4. An induction furnace according to claim 1 wherein the coil means is litz.

5. An induction furnace according to claim 1 wherein a crucible supporting means is provided below said casing for retaining the material to be treated.

6. An induction furnace according to claim 1 wherein means for circulating a cooling agent around said coil means and through said casing is provided.

7. An induction furnace according to claim 1 wherein the coil means is a tubular coil and means are provided I for circulating a cooling agent through said coil.

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8. An induction furnace according to claim 1 wherein induction current insulation means are provided around said casing whereby to prevent passage of current outwardly through the Walls of said chamber.

9. An induction furnace according to claim 1 wherein fireproofing means are provided along the walls of said chamber surrounding said casing.

10. An induction furnace according to claim 1 wherein a pouring outlet for molten metallic material is provided in said furnace chamber.

11. An induction furnace for heating metallic materials under vacuum conditions by means of induction current which comprises a furnace chamber capable of being maintained under vacuum conditions, an induction coil means disposed within said chamber, a metallic casing having an inner and an outer Wall concentric with one another and connected at their upper and lower edges enclosing said coil means therebetween in vacuum sealed condition with respect to said chamber, an open core space being defined by said inner wall constructed and arranged to receive therewithin metallic material to be treated, said casing having a transverse slit opening defined through said inner and outer walls, said slit opening being in vacuum sealed condition with respect to said chamber, and means for supplying induction current to said coil means.

References Cited in the file of this patent UNITED STATES PATENTS 1,849,476 Brace Mar. 15, 1932 1,915,700 Tama June 27, 1933 2,308,945 Van Embden Jan. 19, 1943 2,909,585 Tudbury Oct. 20, 1959 FOREIGN PATENTS 169,771 Switzerland Sept. 1, 1934 

